"Kiln" is a term of art for designating a type of oven used for firing ceramic wares. A ceramic material, such as any member of that class of materials commonly designated as "clay", is wetted into a plastic mass and preshaped prior to being dried and then fired in the kiln into a permanently rigidized or sintered structure. After being subjected to the kiln's firing cycle, a fired object is usually somewhat smaller than its unfired counterpart, resulting in increased strength and density.
The present invention particularly relates to household or hobbyist kilns which are generally constructed much more cheaply than their commercial counterparts. In such kilns, the firing cycle is typically manually stepped by an operator from low to medium to high power levels by an operator controlled switch on a control panel of the kiln.
In order for the ceramic ware to fire properly it is necessary for an operator to manually step the manual switch from low to medium after the switch has been in the low position for an hour. One hour thereafter, the operator must return and step the manual switch from medium to high. Such kilns are typically equipped with a control device that detects when a specific kiln cycle end point temperature is reached. Upon detecting this temperature, a switch is tripped which shuts off the kiln to hopefully prevent or reduce the chance of overfiring.
Great care must taken by an operator to insure that the firing cycle progresses in accordance with power increases from low to medium after one hour at low power and from medium to high after one hour at medium power. In this regard, a ceramic ware prior to a kiln firing run has an undetermined amount of moisture within it. In the low power cycle, the ware is heated from room temperature to below the boiling temperature of water to rid the ware of moisture without boiling. If the moisture is boiled out of the ware, it is likely to crack.
For ceramic wares with a high degree of moisture content, more than hour may be required for this phase of the firing cycle. The shifting of the power level to medium power prior to the moisture being eliminated will initiate boiling and will likely cause cracking of the ware.
The medium power phase in the kiln firing cycle raises the kiln temperature to a point where the pliable clay material is rigidized into quartz crystal. This structural change occurs at a temperature on the order of 600.degree. F.
The high power phase of the kiln firing cycle raises the kiln temperature such that the ceramic ware glaze completely melts to impart the desired color and texture to the finished product. Upon reaching or slightly exceeding the glaze firing temperature, which typically will be above 1200.degree. F., the kiln will automatically shut off.
If an operator forgets to advance the operating cycle from low to medium, the kiln temperature remains at a point which is too low to initiate the quartz conversion phase noted above. Thus, assuming that the ware is moisture free, any excess time spent in the low power cycle translate directly into wasted power and increased operating expenses.
Similarly, any extra time that the kiln is maintained in the medium power phase does not serve to melt the glaze (which occurs in the high power phase) and likewise results in wasted energy and increased expenses. Accordingly, in order to produce a desired high quality ceramic ware in an economic fashion, it is necessary that the firing cycle be advanced on schedule.
In advancing through the firing cycle of a kiln, it is necessary to disconnect relatively large amounts of power. In this regard, some hobbyist kilns may draw as much as 60 amps of current.
Prior attempts to automate sequencing through the hobbyist kiln firing cycle have not been particularly successful. In order to automatically sequence through the high power level firing cycle of such a kiln requires electronic components whose expense cannot be cost justified. For example, in order to disconnect the high power levels in such a kiln would typically require large and expensive triacs. Moreover, in order to properly isolate such an electronic control circuit from the extreme heat generated in the kiln further increases the costs associated with such an electronic implementation. Additionally many electronic components do not perform very well under extreme heat conditions.
In order to circumvent costs associated with disconnecting relatively high power levels in the kiln, electronic control devices have heretofore been designed using a control switch, whereby the duty cycle is switched on for a predetermined percentage of the time and off for the remainder of the cycle to thereby control the power level. The successful operation of such electronic control devices depends on their ability to smoothly simulate a ramp function which gradually increases the power level from a low to a high setting.
Such electronic control devices (due perhaps to the large amount of heat generated within the kiln) have failed to accurately match the kiln firing cycle control achieved by an operator simply manually adjusting a control switch each hour. Correspondingly, the ceramic products generated by such automatically controlled kilns have failed to match the quality of the products generated by the manually operated hobbyist kiln.
The present invention utilizes a motor driven, mechanical switching mechanism which automatically advances the kiln firing cycle through the timing sequence that an operator was heretofore depended upon to move manually. Thus, the switching mechanism of the present invention insures that the kiln firing sequence includes accurately timed low, medium, and high cycles.
The automatic switching mechanism of the present invention allows for the production of high quality ceramic wares while requiring no manual intervention and while insuring that power is not wasted due to an operator's forgetfulness. The present invention also insures that the pottery produced is of a consistently high quality regardless of changes in kiln operators. Additionally, the automatic switching mechanism design of the present invention is not affected by the tremendously high temperatures within the kiln.
The present invention permits an operator to override the preprogrammed firing cycle by manually manipulating a control switch to modify the firing cycle, e.g., to maintain a particular ceramic piece in a predetermined phase of the operating cycle longer (or shorter) than if the kiln cycled automatically. If the switching mechanism is programmed for a relatively long phase of low power operation, an operator can rotate the automatic switching mechanism to shorten the low power phase, as desired.
The exemplary embodiment of the present invention, in addition to properly sequencing the kiln firing cycle, energizes low, medium, and high power level indicators on the kiln control panel to inform the operator as to the cycle the kiln is currently operating in.
The exemplary embodiment of the present invention includes a control panel having a firing cycle initiating control knob. Manipulation of the control knob appropriately supplies power to the kiln's heating elements and energizes a motor. The control knob is mounted on a shaft which is rotated by the energized motor. Also disposed on this shaft are a plurality of cams which rotate and act in concert to interconnect a set of contacts to thereby control the kiln's power level settings. The rotation of the cams also controls the timely energization of control panel low, medium, and high power level indicators.